龙门山南段前陆大川-丹棱剖面磁组构特征及其构造意义

Magnetic Fabric Characteristics of the Dachuan-Danling Profile in the Foreland Fold Belt of the Southern Longmenshan Tectonic Belt and its Tectonic Significance

龙门山南段前陆盆地作为龙门山构造带南段盆-山耦合关系的具体响应,其中-新生代沉积地层的构造变形特征和叠加改造关系在一定程度上反映了龙门山构造带南段的形成与演化。本文基于横跨龙门山南段前陆的大川-丹棱剖面,通过野外构造解析和岩石磁组构特征,对前陆构造变形开展精细研究。构造解析揭示,剖面上发育一系列与双石断裂带走向近平行的宽缓褶皱和浅层逆冲断层,褶皱表现为北西翼较缓,而南东翼相对较陡,轴面略向NW倾的不对称特征。磁组构特征揭示,剖面中岩石具低磁化率和典型的沉积岩三轴磁化率椭球体的特征,磁化率椭球体以扁圆形为主,整体反映出弱变形的特征,与褶皱作用前的平行层缩短(LPS)相关;最大磁化率主轴(Kmax)优势方位为NE-SW向,与龙门山构造带走向平行,主要反映了NW-SE向挤压作用。剖面还发育有磁面理与地层层面斜交、磁线理与地层走向斜交两类非典型磁组构,前者是褶皱作用中层间平行简单剪切的产物,后者则是构造叠加的产物。通过磁组构反映的应变分析结果,厘定出龙门山南段前陆中生代晚期主要受到NW-SE向挤压,与新生代的挤压应力方向呈小角度斜交,且挤压作用所产生的透入性应变到达了熊坡背斜南东翼以东地区,该认识为进一步深入认识龙门山构造带的发展演化过程奠定了基础。

The foreland basin of the southern Longmenshan tectonic belt(SLTB)is the response of the basin mountain coupling in the SLTB.The structural deformation and superposition transformation relationship of the Meso-Cenozoic sedimentary strata in the foreland basin can reflect the formation and evolution of the SLTB.Detailed studies,including field structural analysis and anisotropy of magnetic susceptibility(AMS),were conducted on sedimentary rocks from the Dachuan-Danling profile in the foreland fold belt of the SLTB in order to better understand the tectonic evolution of the SLTB.Field observations reveal that deformations of the foreland fold belt of the SLTB are mainly characterized by NE-SW-trending open fold and shallow thrust faults.The fold is characterized by the asymmetry that the NW limb is relatively gentle and the SE limb is relatively steep,and the axial surface is slightly inclined to NW.Samples of magnetic fabrics from the Dachuan-Danling profile are characterized by the low magnetic susceptibility and triaxial magnetic susceptibility ellipsoids common in sedimentary rocks,and the magnetic susceptibility ellipsoid is mainly oblate.The magnetic fabric of the profile reflects the weak deformation associated with the layer-parallel shortening(LPS)before the folding.The orientation of the maximum magnetic susceptibility principal axis(Kmax)is NE-SW,which is parallel to the strike of the Longmenshan tectonic belt,and mainly reflects the compression in NW-SE direction.There are also the following atypical magnetic fabrics:1)magnetic foliation that is oblique to the bedding and that was associated with layer-parallel simple shearing during folding;2)magnetic lineation that is to varying degrees oblique to the strike of bedding and represents a superposition of structures.In addition,the stress and strain analysis by the magnetic fabric shows that the foreland fold belt of the SLTB was mainly affected by the NW-SE compression stress field in the late Mesozoic,which is oblique to the Cenozoic stress direction at a small angle.The penetrating strain(PES)produced by compression reaches a few kilometers east of the SE limb of the Xiongpo anticline.This finding provides the basis for a comprehensive understanding of the evolution and development of the Longmenshan tectonic belt.